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The fast track to the core


Corus has used technology developed for the security sector to create a prefabricated core system that can be erected six times faster than traditional methods.

Joanna Booth went to see Corefast in action

WHEN the building is complete it will look like many another student accommodation block: a series of low-rise buildings clad in brickwork surrounding an 18-storey tower glinting with zinc and glass.

This type of project is bread and but ter to main contractor Ocon Construction. The company, founded just two years ago, has already leapt into the top 50 contractors league on the back of a rash of private housing and student accommodation work.

But there is something new and experimental about Birmingham One. It is pioneering Corefast, a new system developed by steel maker Corus, which allows for speedy construction of cores and stairwells.

Corefast is a development of Corus's Bi-Steel product, which does away with the need for traditional concrete shuttering. Two steel panels are joined by a grid of steel bars.

The panels, which can be connected into L-, C- or T-shaped modules, are craned into place on site. Once bolted together, the gap between the steel sheets is filled with structural-grade concrete.

The system has now been developed for cores, lift shafts and stairwells, and Corus claims the erection time is just 15 per cent of the time it would take to put up a core using traditional methods.

Corefast has been used on three projects before, but at 18 storeys this is by far the highest rise it has achieved.

The initial suggestion to use the system came from engineering consultancy Henry Smith, responsible for all the steelwork on the project.

'We had worked with it previously and found it very successful, ' says Gerard Kitching, managing director of Henry Smith. 'We recommended it to Tier Consult, the structural engineer doing the design.'

The project had originally been designed with concrete cores and sheer walls, but the team believed Corefast could bring significant benefits to cost and programme.

'We worked up two bids, one using concrete cores and an alternative using Corefast, with a separate price and programme, ' Mr Kitching says. 'Ocon wanted to judge the full extent of its effects.'

Steve Dando, Ocon Midlands construction director, says: 'It became evident that Corefast was the right choice. It saves time, and that's money. Not to mention all the other benefits. As a company we are keen to embrace innovation - otherwise you get left behind.'

The other benefits Mr Dando refers to are manifold.

As well as being six times faster to erect than an equivalent concrete core, the large element of off-site prefabrication gives much greater efficiency, reducing the amount of handover between trades and minimising site congestion.

The greater speed of erection allows overall project costs to be lowered. Because Corefast walls are slimmer than the traditional equivalent, this releases lettable f loor space, increasing the capital value of the project.

Connectors are welded into the Corefast in the factory, which gives a built-in accuracy between the steel-to-steel connections to the building's frame.

Safety also benefits - with no need for formwork there is a large reduction in the amount of hours spent working at height.

A smaller amount of Corefast is required to achieve the same loading capacities as concrete core.

'We were able to lose extra cores and move traditional vertical bracing, ' Mr Kitching says. 'The stair void, lift shaft, service risers and doors are all contained within the Corefast structure.'

The £20 million contract will finance the build of 604 student bedrooms in a group of buildings with one tower reaching to 18 storeys, and will also include leisure facilities and a swimming pool.

It is located right in the centre of Birmingham, and access is restricted. Nothing can be delivered in lengths of more than 15 m and space is at a premium, so panels arrive just in time.

Laid over 65 piles, the 1.5 m-thick pile cap has starter bars for the Corefast system set into it. The first section of core to be installed is four storeys and 15 m high, and each modular section is covered in bitumen paint below ground floor level. Each section is lowered in using two cranes, as lifts are difficult due to the size and shape of modules.

The sections are levelled by holding down bolts around the perimeter. Above the core level an extra metre of Bi-Steel is left to protrude to act as a guardrail around the voids the modules create. The panels are then filled with concrete.

'It takes five days to do the whole first section, ' Mr Dando explains. 'It could be five or six weeks using concrete cores.'

The structural steel is erected up to the fourth storey and bolted onto the pre-welded connections on the Corefast.

'The beams are pre-drilled by Corus with captive nuts on the back, ' Mr Dando says. 'It would be a nightmare with a concrete core.'

Decks are then poured across the first four levels before another two storeys of Corefast are bolted on and the process repeated until the desired height is reached.

'We've been very happy with its progress, ' Mr Dando says. 'We'll be using it on futu re projects.'

Developing Bi-Steel

BI-STEEL panels are formed by joining two steel plates with a spacer bar. Plate thicknesses run from 6 mm to 20 mm and bar lengths from 200 mm to 700 mm. The maximum panel size available is 18 x 2 m. Face plates can be pre-bent to create curved panels.

Bi-Steel has been around for a few years, predominantly used in for security applications because of its high level of blast resistance. It has also been used on foundations for offshore wind turbines, on offshore oil and gas structures and for anti-attack vehicle barriers.

But the Corus team realised the product, prefabricated into a modular kit, could be used to form cores in any building.

Corus says the composite gives great performance, with the steel faceplates resisting in-plane and bending forces and the concrete core resisting compressive and shear forces. The connecting rods joining the plates provide shear reinforcement to the core.

The panels have significant stiffness and strength, but are more ductile than reinforced concrete - hence their suitability for blast-resistant structures.

The first project to use the new modular product was Dundrum Cinema in Dublin, where John Sisk and Son erected the world's first modular composite lift core in May 2004. The six-storey core was installed in five days. In 2005, Beechwood Developments used Corefast on phase one of Westpoint Homes' Pollockshaws Road housing project in Glasgow, tak ing seven days to erect four cores. The traditional equivalent would have taken six weeks.

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